Bimodal behavior of accretion disks - Theory and application to Cygnus X-1 transitions

Abstract

A theory of Newtonian accretion disks is developed based on an explicit formulation of viscous stresses arising from turbulent magnetic fields which are generated by Keplerian differential rotation of plasmas. It is particularly pointed out that two physically distinct states exist in the middle part of the accretion disk surrounding a black hole; a transition from one state to the other takes place when the physical conditions of the accreting gas near the disk outer boundary cross over the critical conditions separating the two states. The thermal stability of those states is examined. It is also found that within a certain inner radius, the disk develops into a single high-temperature state, irrespective of the critical conditions; copious emission of hard X-rays arises from this inner domain. These results are applied to construct a definite numerical model of Cyg X-1 accounting for observed features of its transitions, such as magnitudes and spectral distributions of the X-ray luminosity, transition times, and variabilities. The possibility of transient X-ray sources associated with accretion onto black holes is predicted.